Aerosols for the delivery of therapeutic agents to the respiratory tract have been described, for example, Adjei, A. and Garren, J., Pharm. Res., 7: 565-569 (1990); and Zanen, P. and Lamm, J.-W. J. Int. J. Pharm., 114: 111-115 (1995). The respiratory tract encompasses the upper airways, including the oropharynx and larynx, followed by the lower airways, which include the trachea followed by bifurcations into the bronchi and bronchioli. The upper and lower airways are called the conducting airways. The terminal bronchioli then divide into respiratory bronchioli which then lead to the ultimate respiratory zone, the alveoli, or deep lung. Gonda, I. “Aerosols for delivery of therapeutic and diagnostic agents to the respiratory tract,” in Critical Reviews in Therapeutic Drug Carrier Systems, 6: 273-313 (1990). The deep lung, or alveoli, are the primary target of inhaled therapeutic aerosols for systemic drug delivery.
Inhaled aerosols have been used for the treatment of local lung disorders including asthma and cystic fibrosis (Anderson, Am. Rev. Respir. Dis., 140: 1317-1324 (1989)) and have potential for the systemic delivery of peptides and proteins as well (Patton and Platz, Advanced Drug Delivery Reviews, 8: 179-196 (1992)). However, pulmonary drug delivery strategies present many difficulties for the delivery of macromolecules; these include protein denaturation during aerosolization, excessive loss of inhaled drug in the oropharyngeal cavity (often exceeding 80%), poor control over the site of deposition, lack of reproducibility of therapeutic results owing to variations in breathing patterns, the frequent too-rapid absorption of drug potentially resulting in local toxic effects, and phagocytosis by lung macrophages.
Considerable attention has been devoted to the design of therapeutic aerosol inhalers to improve the efficiency of inhalation therapies. Timsina et. al., Int. J. Pharm., 101: 1-13 (1995); and Tansey, I. P., Spray Technol. Market, 4: 26-29 (1994). Attention has also been given to the design of dry powder aerosol surface texture, regarding particularly the need to avoid particle aggregation, a phenomenon which considerably diminishes the efficiency of inhalation therapies. French, D. L., Edwards, D. A. and Niven, R. W., J. Aerosol Sci., 27: 769-783 (1996). Dry powder formulations (“DPFs”) with large particle size have improved flowability characteristics, such as less aggregation (Visser, J., Powder Technology 58: 1-10 (1989)), easier aerosolization, and potentially less phagocytosis. Rudt, S. and R. H. Muller, J. Controlled Release, 22: 263-272 (1992); Tabata, Y. and Y. Ikada, J. Biomed. Mater. Res., 22: 837-858 (1988). Dry powder aerosols for inhalation therapy are generally produced with mean geometric diameters primarily in the range of less than 5 μm. Ganderton, D., J. Biopharmaceutical Sciences, 3: 101-105 (1992); and Gonda, I. “Physico-Chemical Principles in Aerosol Delivery,” in Topics in Pharmaceutical Sciences 1991, Crommelin, D. J. and Midha, K. K., Eds., Medpharm Scientific Publishers, Stuttgart, pp. 95-115, 1992. Large “carrier” particles (containing no drug) have been co-delivered with therapeutic aerosols to aid in achieving efficient aerosolization among other possible benefits. French, D. L., Edwards, D. A. and Niven, R. W., J. Aerosol Sci., 27: 769-783 (1996).
The human lungs can remove or rapidly degrade hydrolytically cleavable deposited aerosols over periods ranging from minutes to hours. In the upper airways, ciliated epithelia contribute to the “mucociliary escalator” by which particles are swept from the airways toward the mouth. Pavia, D. “Lung Mucociliary Clearance,” in Aerosols and the Lung: Clinical and Experimental Aspects, Clarke, S. W. and Pavia, D., Eds., Butterworths, London, 1984. Anderson, Am. Rev. Respir. Dis., 140: 1317-1324 (1989). In the deep lungs, alveolar macrophages are capable of phagocytosing particles soon after their deposition. Warheit, M. B. and Hartsky, M. A., Microscopy Res. Tech., 26: 412-422 (1993); Brain, J. D., “Physiology and Pathophysiology of Pulmonary Macrophages,” in The Reticuloendothelial System, Reichard, S. M. and Filkins, J., Eds., Plenum, New York, pp. 315-327, 1985; Dorries, A. M. and Valberg, P. A., Am. Rev. Resp. Disease 146: 831-837 (1991); and Gehr, P., Microscopy Res. and Tech., 26: 423-436 (1993). As the diameter of particles exceeds 3 μm, there is increasingly less phagocytosis by macrophages. Kawaguchi, H., Biomaterials 7: 61-66 (1986); Krenis, L. J. and Strauss, B., Proc. Soc. Exp. Med., 107: 748-750 (1961); and Rudt, S. and Muller, R. H., J. Contr. Rel., 22: 263-272 (1992). However, increasing the particle size also has been found to minimize the probability of particles (possessing standard mass density) entering the airways and acini due to excessive deposition in the oropharyngeal or nasal regions. Heyder, J., J. Aerosol Sci., 17: 811-825 (1986).
Local and systemic inhalation therapies can often benefit from a relatively slow controlled release of the therapeutic agent. Gonda, I., “Physico-chemical principles in aerosol delivery,” in: Topics in Pharmaceutical Sciences 1991, D. J. A. Crommelin and K. K. Midha, Eds., Stuttgart: Medpharm Scientific Publishers, pp. 95-117 (1992). Slow release from a therapeutic aerosol can prolong the residence of an administered drug in the airways or acini, and diminish the rate of drug appearance in the bloodstream. Also, patient compliance is increased by reducing the frequency of dosing. Langer, R., Science, 249: 1527-1533 (1990); and Gonda, I., “Aerosols for delivery of therapeutic and diagnostic agents to the respiratory tract,” in Critical Reviews in Therapeutic Drug Carrier Systems 6: 273-313 (1990).
Controlled release drug delivery to the lung may simplify the way in which many drugs are taken. Gonda, I., Adv. Drug Del. Rev., 5: 1-9 (1990); and Zeng, X., et al., Int. J. Pharm., 124: 149-164 (1995). Pulmonary drug delivery is an attractive alternative to oral, transdermal, and parenteral administration because self-administration is simple, the lungs provide a large mucosal surface for drug absorption, there is no first-pass liver effect of absorbed drugs, and there is reduced enzymatic activity and pH mediated drug degradation compared with the oral route. Relatively high bioavailability of many molecules, including macromolecules, can be achieved via inhalation. Wall, D. A., Drug Delivery, 2: 1-20 1995); Patton, J. and Platz, R., Adv. Drug Del. Rev., 8: 179-196 (1992); and Byron, P., Adv. Drug. Del. Rev., 5: 107-132 (1990). As a result, several aerosol formulations of therapeutic drugs are in use or are being tested for delivery to the lung. Patton, J. S., et al., J. Controlled Release, 28: 79-85 (1994); Damms, B. and Bains, W., Nature Biotechnology (1996); Niven, R. W., et al., Pharm. Res., 12(9): 1343-1349 (1995); and Kobayashi, S., et al., Pharm. Res., 13(1): 80-83 (1996).
Drugs currently administered by inhalation come primarily as liquid aerosol formulations. However, many drugs and excipients, especially proteins, peptides (Liu, R., et al., Biotechnol. Bioeng., 37: 177-184 (1991)), and biodegradable carriers such as poly(lactide-co-glycolides) (PLGA), are unstable in aqueous environments for extended periods of time. This can make storage as a liquid formulation problematic. In addition, protein denaturation can occur during aerosolization with liquid formulations. Mumenthaler, M., et al., Pharm. Res., 11: 12-20 (1994). Considering these and other limitations, dry powder formulations (DPF's) are gaining increased interest as aerosol formulations for pulmonary delivery. Damms, B. and Bains, W., Nature Biotechnology (1996); Kobayashi, S., et al., Pharm. Res., 13(1): 80-83 (1996); and Timsina, M., et al., Int. J. Pharm., 101: 1-13 (1994). However, among the disadvantages of DPF's is that powders of ultrafine particulates usually have poor flowability and aerosolization properties, leading to relatively low respirable fractions of aerosol, which are the fractions of inhaled aerosol that escape deposition in the mouth and throat. Gonda, I., in Topics in Pharmaceutical Sciences 1991, D. Crommelin and K. Midha, Editors, Stuttgart: Medpharm Scientific Publishers, 95-117 (1992). A primary concern with many aerosols is particulate aggregation caused by particle-particle interactions, such as hydrophobic, electrostatic, and capillary interactions. An effective dry-powder inhalation therapy for both short and long term release of therapeutics, either for local or systemic delivery, requires a powder that displays minimum aggregation, as well as a means of avoiding or suspending the lung's natural clearance mechanisms until drugs have been effectively delivered.
Therefore, a need exists for dry-powders suitable for inhalation which minimize or eliminate the above-mentioned problems.